Transducer and vibratory diaphragm



Dec. 27, 1960 H. s. KNOWLES 2,966,558

TRANSDUCER AND VIBRATORY DIAPHRAGM Filed Oct. 11, 1956 3 Sheets-Sheet 1x/ i3 flLCii P1 18 6 g 10 4 u 14 19 i 2 22 20 E o -2 8 we 16' 3 i8 -s Qa k 28 F'RE'QUENCY (/f/LOCyCl-ES PER 5ECOND Dec. 27, 1960 H. s. KNOWLES2,966,558

TRANSDUCER AND VIBRATORY DIAPHRAGM Filed Oct. 11, 1956 3 Sheets-Sheet 2INVENTOR.

BYTA/U/Q Dec. 27, 1960 H. s. KNOWLES 2,966,558

TRANSDUCER AND VIBRATORY DIAPHRAGM Filed Oct. 11, 1956 5 Sheets-Sheet 3.120 ;2Z I E r fi a I M INVENTOR. W fi zawzs United States Patent budTRANSDUCER AND VIBRATORY DIAPHRAGM Hugh S. Knowles, Glen Ellyn, Ill.(9400 Belmont Ave., Franklin Park, Ill.)

Filed Oct. 11, 1956, Ser. No. 615,406

21 Claims. (Cl. 179-114) This invention relates toelectro-acoustictransducers, and particularly to miniature transducers for use asmicrophones in hearing aids. It also relates to the combination of thetransducer and a hearing aid housing and to a new vibratory diaphragmwhich contains in its structure a Thuras or inertance tube. In thisapplication, the term electro-acoustic transducer identifies a caseproviding a motor or back cavity, a motor in the cavity, a diaphragm, adrive connection between the motor and the diaphragm and a lid over thediaphragm to form a front cavity. A transducer is shown in Figures 4,and 6. In Figure 1 a transducer is schematically shown in a hearing aidhousing 12 which contains besides the transducer 10, batteries,transistors, etc.

Applicants general object is to provide a miniature electro-accoustictransducer having the improved response in the lower frequenciesprovided by a Thuras tube while limiting the functioning of that tube tothe transducer unit itself. Thuras taught in United States LettersPatent No. 1,869,178, dated July 26, 1932, that by venting the backcavity of an electro-accoustic transducer to the open air at a pointadjacent to the diaphragm, the response of the diaphragm to lowfrequency sound, i.e., 50200 cycles, would be improved.

Hearing aid manufacturers, most of whom do not manufacture the delicatetransducers, have followed Thuras teaching only in part, and thisbecause Thuras taught that his tube must be vented to open air or atleast to a large volume of air. While this teaching could be used inordinary communication equipment, hearing aid manufacturers were obligedto place a transducer inside an attractive housing with the diaphragmconcealed, and thereby they created a substantially closed outer cavityinto which Thuras taught his tube could not be vented.

Thus, in Figure 1, applicant schematically shows a transducer unit 10inside a sealed hearing aid housing 12, with a Thuras tube 14 ventingthe back cavity 16 into the chamber 18. The numeral 19 identifies thediaphragm and 20 is a rubber gasket positioned between the transducer 10and the inside wall of the housing 12 having a small opening 21 for theentrance of sound. This assembly creates a front cavity 22, almostclosed, into which the Thuras tube is not to be vented if the teachingof Thuras is to be followed. Hence the tube is located as shown, and thechamber 18 bears no relation to the sound receiving side of thediaphragm, i.e., to the cavity 22 or even in the outside space in frontof the opening 21.

This construction produces, referring to Figure 2, the responseindicated by the dot-dash line 26. While the response is god in thelower frequencies, it is followed by a low valley 15. This peak andvalley result from resonance characteristics of the chamber 18, and fromthe fact that there is no air path between the motor cavity 16 and thesound side of the diaphragm. In this closed chamber 18 design, thehearing aid manufacturers can make the microphones uniform, but the poorresponse in the 450900 cycle range is unsatisfactory. 1

Still other manufacturers have provided ports or open- 2,966,558Patented Dec. 27, 1960 ings such as 28, see Figure 3, in the chamber 18which will allow the sound vibrations to escape from the chamber 18 andreach the front side of the diaphragm. Commonly, this passageway 28 isadjacent or around the volume control of the hearing aid. Thisconstruction may produce a response such as that indicated by the dottedgraph 30 in Figure 2. This design is superior to that of Figure 1. Ithas, however, several drawbacks. Firstly, tests on a run of thesehearing aid microphones show a lack of uniformity in response,indicating that it is difiicult to assemble uniformly the cases insidethe hearing aid housings. Also, the air through the opening 28 has sucha long and uncertain path in order to reach the face of the diaphragmthat the benefit of the Thuras tube is largely lost. Again, the ports 28provide a path for acoustic feedback at other frequencies and permitdust to reach the back cavity of the transducer.

It should be mentioned that leaks between improperly assembled matingparts of a hearing aid housing produce an effect similar to thatobtained by the deliberately positioned port 28.

Also, in order to protect the diaphragm, transducer manufacturers haveplaced a cover with small center hole over the diaphragm, therebycreating a substantially closed front cavity, as illustrated in Figure 4where the cover bears numeral 36. This entire assembly is sealed and thehearing aid manufacturer simply assembles it in a housing such as 12 inFigure 1 with the sound opening 58 of the transducer in alignment withor close to the opening 21 in the housing 12.

The first specific object of this invention is to connect a Thuras tubedirectly between the closed back cavity 16 and the semi-closed frontcavity 22, see Figure l, of a hearing aid microphone, and particularlybetween the back cavity 44 and the front cavity 54 of Figure 4. Thiswould make the electro-acoustic transducer a completely self-sufficientunit, affected by air only within its walls and capable of final testingat the transducer plant Where there are equipment and workmen capable ofadjusting all transducers to a uniform response. Applicant set out tore-examine Thuras fundamental principle that has tube should be ventedinto an open space on the sound receiving side of the diaphragm, andfound that a Thuras tube would function between a closed back cavity anda substantially closed front cavity, i.e., having an opening or openingswhose total area is much less than the area of the diaphragm.

The next specific object of this invention is to incorporate the Thurastube inside a standard hearing aid microphone case. Manufacturers ofthese electro-acoustic transducers house the internal working parts insmall metallic cases that may be a cylinder or a right parallelpiped.These cases may have dimensions of the order of 5 x A x /2 inch. Twodimensions of the case are determined by the two major dimensions of thediaphragm and the other dimension of the case is determined by thethickness of the transducer motor. Applicant wishes to incorporate aThuras tube of requisite volume within the planes of the outer surfacesof the diaphragm and motor as projected to form a regular case such as aclosed cylinder or right parallelpiped.

In attaining this object, the first feature is the positioning of theThuras tube through the vibrating portion of the diaphragm itself, i.e.,inside the clamped configuration, with the tube in the vacant spaceusually available in the back cavity. In attaining this feature,applicant positions the tube at a point through the diaphragm which doesnot participate appreciably in the vibratory movements.

.The second general object of this invention is to mount a Thuras tubeor a pluralityof Thuras tubes on the vibratory portion of the diaphragmitself. In all applications, space limitations are critical, and thevolume of the Thuras tube and the relationship of its diameter to itslength are dependent upon the relative sizes of the front and backcavities. There are several embodiments of these Thuras tube diaphragms.In each, it is essential that the existence of the Thuras tube on thediaphragm not interfere with the proper functioning of the diaphragm,and it is therefore dmirable that the Thuras tube or tubes be balancedwith respect to the center of vibration of the diaphragm.

In a preferred embodiment, the Thuras tube is incorporated in the driverod that connects the center of the diaphragm to the armature of themotor. in another embodiment, four short Thuras tubes are mounted 90apart around the center of the diaphragm and substantially at rightangles to its surface. In this embodiment, each Thuras tube projectsabove one or both surfaces of the diaphragm, and this requires cavitiesof suitable depth.

Several embodiments are based upon the Thuras tube lying in the plane ofthe diaphragm. This makes possible arcuate and spiral tubes of greatlength and small cross section. In one of these embodiments, tubes aremounted on the face of the diaphragm. A preferred construction employs adouble layer diaphragm wherein the Thuras tubes are formed byfacinggrooves in the two components. In one embodiment, the grooves areradial. In another embodiment, the grooves are spiral. The two spiralThuras tubes are of great length and small cross section, therebypermitting the use of extremely shallow cavities on both sides of thediaphragm.

These, and such other objects and features as may hereinafter appear,are attained in the embodiments of the invention shown in theaccompanying drawings, wherein: The relative depth of the grooves in thediaphragm components is so related to the convex side of the diaphragmthat that portion of the Thuras tube near the diaphragms center liespredominantly on the concave side of the diaphragm, whereas that portionnear clamped periphery lies predominantly on the convex side, therebyutilizing the available space most advantageously.

There are applications where it is not practicable to locate the Thurastube on the diaphragm so that the diaphragm is balanced around itsvibrational center. It is a feature of this invention.

Figure l is a schematic cross section of an electroacoustic transducermounted within a sealed chamber;

Figure 2 is a graph showing the response of applicants directlyconnected Thuras tube compared with the response of Thuras tubes openinginto a sealed chamber and into an unsealed chamber;

Figure 3 is a cross-sectional, schematic illustration of anelectro-acoustic transducer mounted in an unsealed chamber of a hearingaid;

Figure 4 is a longitudinal section through an electroacoustic transducerdisposed in its case with a fixed 'Iburas tube;

Figure 5 is a View taken on the iine 5-5 of Figure 4;

but partly cut away;

Figure 7 is a perspective view of a diaphragm and a tubular drive rodwhich constitutes the Thuras tube, a preferred form of applicantsinvention; a

Figure 8 is a sectional view taken on the line S@ of Figure 7;

Figure 9 is a sectional view of a modified form of the embodiment ofFigures 7 and 8, wherein the opening through the cover is out ofalignment with the tubular drive rod;

Figure 10 is a top view of. the portion of the case and diaphragm ofFigure 9;

Figure 11 is a perspective view of a circular diaphra .n with fourThuras tubes mounted therein;

Figure 12 is a view taken on the line 32ll2 of Figure l1;

Figure 13 is a perspective view of a rectangular diaphragm, such as thatin the embodiment shown in Figures 46, but composed of two sheets ofvibratable material with the Thuras tubes in arcuate curves betweenthem;

Figure 14 is a View taken on the line 14-14 of Figure 13;

Figure 15 is a plan view of a circular diaphragm showing two balancedspiral Thuras tubes on a circular diaphragm;

Figure 16 is a perspective view of a rectangular diaphragm having fourradiating Thuras tubes; and

Figure 17 is a sectional view taken on the line 1717 of Figure 16.

Continuing to refer to the drawings, particularly Figures 4, 5 and 6,the numeral 32 identifies the case of a microphone over the open face ofwhich is first positioned a bulkhead 34 carrying a diaphragm 46, andthen a lid 36. The bulkhead 34 serves as the base for all operatingparts of the transducer, and supports on its iower side a motor 38 and aThuras tube 41 The bulkhead 34 has a reverse flange 42 around itsperimeter and forms a cavity 44. Sealed to the outer face of theperipheral flange 42 is the diaphragm 46, t0 the vibratory center ofwhich is anchored the armature drive rod 48. The drive rod 43 passesthrough an opening St) in the bulkhead 3d and drives an armature t? Thisopening constitutes the air communication between the back cavity 52 andthat portion of the back cavity between the bulkhead 34 and thediaphragm 46. The recess 44 is the important part of the back cavity,but the back cavity includes the large space 52 which houses the motorbecause of the passageway 50 in the bulkhead 34. The front cavity isformed by the peripheral reverse flange 55 on the lid 36. The soundopening is centrally positioned at 58.

Referring to Figure 5, the diaphragm 46 carries a lozenge-shaped channel47 which substantially confines vibrations within its boundaries,leaving the corner areas such as 43 comparatively rigid and thereforeusable for the port to the edges of which the Thuras tube 40 may besealed. The channel 47 is formed by two reverse bends. A single bend orparallel grooves may be used. In the embodiment shown in these figures,the corner area 55 of the bulkhead 34 is raised to the plane of theouter surface of the flange and the diaphragm lies flushly upon it.

For purposes of this invention, the motor need not be described. It hasbeen shown in sufiicient detail to present clearly a vibratable armatureand drive rod because, as will appear hereinafter, a hollow drive rodmay be used as the Thuras tube.

Means for computing the diameter and length of the T huras tube 40 willnot be set forth as these formulas are well known to those in the artand diflicult to apply to a particular structure. They do not affect theconcept of positioning a Thuras tube between a closed cavity and onethat is almost closed, particularly in a microphone. Moreover, thediameter and length of the tube, its position in the back cavity, andthe position of its outlet in that cavity, are usually determined as aresult of thoughful experimenting. To determine the Thuras tubespecifications by formula, every obstacle presented by the motor to airmovement and resonance of the cavities, as well as the weight andphysical characteristics of the drive rod and armature, must be givensome value. The selection of tube specifications by experimenting withtubes of different lengths and diameters, and testing to produce graphssuch as those in Figure 2, is the practical procedure.

Applicants microphone operates as follows: The sound enters the port 58in the lid 36 and vibrates the diaphragm 46. The diaphragm drives thearmature 49 which varies a sustained magnetic field in which ispositioned a coil and thereby generates a current in the conductors 51and 53.

At frequencies below point a in Figure 2 the inertia of the air in theThuras tube is so low that the difference in pressure on the two sidesof the diaphragm is less than it would be in the absence of the tubebecause relatively rapid pressure equalization occurs through the tube.At frequencies above b the inertia of the air in the tube is so highthat there is negligible displacement of the air through the tube andthe pressure difference between the diaphragm sides is substantiallywhat would exist without the tube. Between frequencies a and b theinertia of the tube air operating in combination with the stiflness ofthe air in the rear cavity (and to a very small extent the front cavity,if any) results in a time lag in the tube air displacement such that thepressure on the rear of the diaphragm is less and on the front of thediaphragm is greater than it would be without the tube.

Thuras tubes mounted on the vibratory portion of the diaphragm Whileapplicants basic idea is shown in Figrues 4, 5 and 6, where the Thurastube connects a substantially closed front cavity with a closed rearcavity, most of the Thuras tubes shown herein are in fact mounted on thevibratory portion of the diaphragm. These are set forth in Figures 7through 17. In these embodiments of the invention, only the diaphragmwith the mounted Thuras tube is shown in the drawings and describedindetail. In each instance, however, one is to consider the specificdiaphragm being described as substituted for the diaphragm 46 in Figures4, 5 and 6 so that its operation will be considered with the semi-closedfront cavity 54. Indeed, all of the embodiments about to be describedshould be considered where the lid 36 is not used but an equivalentresult is obtained by mounting the transducer case against the insidewall of the hearing aid assembly so as to produce a substantially closedfront cavity which is the equivalent of cavity 54 in Figure 4.

A Thuras tube as the drive rod Referring for the moment to Figure 4, thedrive rod 48 connects rigidly the center of vibration of the diaphragm46 and a vibratable portion of the armature 49. The armature-driverod-diaphragm assembly may be widely varied and does not affect theThuras tube construction. In Figures 7 and 8, applicant utilizes aThuras tube as a drive rod. The circular diaphragm is of the type thatwould be used in association with the electro-acoustic transducer shownin copending application Serial No. 436,416, filed June 14, 1954.

Continuing to refer to Figures 7 and 8, 70 is a circular diaphragm,intended to be fastened and sealed at its peripheral edge to a fixedmember which will form a closed cavity similar to 44 and 52 in Figure 4,and with a lid a semi-closed cavity, such as 54 in Figure 4. A tube 72,having a flared end 75, is fastened either as shown in Figure 8, ordirectly to the under side of the diaphragm 78. To the bottom of thetube is affixed a yoke 74 having a transverse channel into which may beslipped the armature 62.

Returning to Figure 4, the point where the yoke 74 of Figures 7 and 8would be attached to the drive rod 72 is at about 76. This determinesthe length of the Thuras tube. Approximately one-half of the entirethickness of the transducer can be utilized as a Thuras tube. The Thurastube of Figures 7 and 8 vents directly into the opening 58 from outsidethe transducer. Alternatively this opening may be at a point remote fromthe drive rod, or the opening 58 of Figure 4 may be moved to a pointremote from the center of the diameter. Where the applicant wishes toavoid venting the Thuras tube into the sound opening 58, a constructionsuch as that shown in Figures 9 and 10 may be used. Here, the upper endof the tube '72 has, been greatly expanded and fastened to the lowersurface of the diaphragm 76. A circle of holes 78 has been cut throughan otherwise solid diaphragm. In either case, applicant is able toutilize space for the Thuras tube which presently exists and can easilybe enlarged without enlarging the outer dimensions of the case.

Thuras tubes at right angles to the general plane of the diaphragmAnother embodiment of the invention showing T huras tubes at rightangles to the surface of the diaphragm is that presented in Figures 11and 12. Here, the diaphragm with depressed channel 82, carries fourThuras tubes 84, 86, 88 and 90, exactly spaced and centered around thediaphragm, the entire assembly being carefully balanced. In thisconstruction, the Thuras tubes require deepening of the front and backcavities. A Thuras tube must contain a definite amount of air withrespect to its diameter. A mere opening through the diaphragm will notfunction as a Thuras tube. The structures shown in Figures 11 and 12,therefore, are useful in those applications where the thickness of thecase is not important.

Thuras tubes lying in the plane of the diaphragm In the remaining fivefigures, Figures l3l7, applicant shows three diaphragms wherein theThuras tubes lie in the plane of the diaphragms. A preferred embodimentis shown in Figures 13 and 14. Here, the diaphragm is composed of twometallic sheets 91 and 92. A lozenge-shaped channel 94 in sheet 91 tendsto limit vibrations of the diaphragm to points within itself. The sheet91 alone is clamped around its periphery to the wall forming one or bothof the cavities. The sheet 92 is substantially stiffer than sheet 91.Sheets 91 and 92 have formed in them complementary, arcuate grooves 96,97, 98 and 99. These grooves are exactly related to each other about thevibratory center 180. The grooves 96 and 97 open into the upper side ofthe diaphragm at 181 and to the lower side at 102; and the grooves 98and 99 open into the upper side of the diaphragm at 103 and to the lowerside at 104. These two grooves can be lengthened so as to have a lengthof substantially of arc. The passageway instead of being oval shaped asshown in Figure 14 may be substantially rectangular in shape.Importantly, the grooves may be in one sheet only, preferably theheavier sheet 92.

Referring to Figure 15, applicant shows the plan view of a circulardiaphragm in which there are two spiral Thuras tubes 186 and 108. Thetube 106 opens to the upper part of the diaphragm 105 at 118 and to thelower side at 112, while tube 108 opens to the upper side at 114 and tothe lower side at 116. These tubes could be lengthened by reducing theirwidth.

In Figures 16 and 17, applicant shows radially directed Thuras tubes120, 122, 124 and 126. These tubes may open directly into the channel128. They may be useful in applications where it is desirable tointroduce the air into the front cavity at points near its periphery,while introducing the air to the other cavity at points near its center,as for example, near the drive rod which passes through a small openingin the bulkhead between the shallow cavity behind the diaphragm and themotor cavity.

While the Thuras tubes lying parallel to the diaphragm have been shownas formed between a two-wall diaphragm, separate tubes mounted on top oron the bottom of the diaphragm may be employed.

With respect to the diaphragms, their peripheral configuration is notimportant sofar as this invention is concerned. Ordinarily, thediaphragm is clamped at its periphery between two holding members. Inapplicants construction of Figures 4-6, it is clamped between thereverse flange 56 of the lid 36 and the reverse flange 42 of thebulkhead 34, although in assembling, it is sealed by adhesive to thereverse flange 42 before the lid 36 is placed on the unit. Additionally,the lids are sealed on the units so that the hearing aid manufacturerdoes not open the unit.

The configuration of the vibratory portion of the diaphragm is notimportant. Thus, a diaphragm having a general rectangular area such asthat shown in Figure 13 may have a circular bend to limit vibration tothe area within the circle, and the Thuras tubes could be mounted asshown in Figure or Figure 7. The area outside the bend consitutes a sortof fixed partition and is useful for mounting purposes.

Referring to Figure 4, the sound opening 58 of the microphone isconcentric with the axis 59 which is normal to the vibratable center ofthe diaphragm and is the axis of the drive rod 48. Better response isobtained if the sound vibrations received from the outside are evenlydistributed around the vibratable center of the diaphragm, but hearingaid manufacturers do not always directly present the opening 58 to theoutside air and hearing aid users often position the microphone under acoat or lapel so that the sound vibrations reaching the opening 53 arenot evenly distributed across it. Consequently, it should be understoodthat this opening can be moved off center, i.e., ofi the center of thediaphragm and still obtain a very satisfactory result.

Venting the opening of the Thuras tube directly into the sound opening58 of the lid 36 may also be undesirable. Applicants test indicatesfirstly, that the Thuras tube should be vented through the diaphragm atpoints out of alignment with the sound opening of the case, andsecondly, that the venting should take place under such circumstancesthat it is distributed over the diaphragm, as is accomplished by thestructures shown in Figures 9 and 10.

The presence of structural obstacles in the motor cavity is not oftheoretical importance. Referring to Figure 4, the principal obstaclefor air movements is the bulkhead 34 which is shown with just oneopening 50. Where the Thuras tube or tubes are mounted on the diaphragm,for all practical purposes, the main motor cavity 52 plays no part, andthe opening 50 need only be suflicient to pass the drive rod 48. Wherethe drive rod Thuras tube such as 72 in Figure 8 is used, the opening 50is necessary, but its proximity to the lower opening of the drive rodtube is helpful.

Summary .All of the Thuras tubes described open into a small frontcavity. The theory of the Thuras tube is that in an intermediate rangeof frequencies, the air in the tube tends to move in or out of the tubeand thereby to reinforce a sound impulse. It is believed that theconfined cavity itself acts as a sort of extension of the Thuras tube soas to slightly change the volume in the cavity and distribute theslightly altered pressure over the entire face of the diaphragm. Usingthe drive pin as a Thuras tube is particularly desirable because itdelivers the air at the vibratory center of the diaphragm. The graph inFigure 1 indicates the actual frequencies in which the Thuras tube isuseful for microphones for hearing aids.

It will also be appreciated that where the Thuras tube is mounted on thediaphragm, problems of attaining exact symmetry so as to secure dynamicbalance become great. In some instances, this balance Will be obtainedby adding weight to a point on the diaphragm which will attain suchbalance.

On page 16 of the 1943 edition of Dynamical Analogies, D. Van NostrandCompany, inc, New York, New York, Harry Ferdinand Olson definesinertance as follows:

Inertance may be expressed as ,1, 'm LiI" Z where m=mass, in gramss=cross sectional area in square centimeters, over which the drivingpressure acts to drive the mass. He gives the formula for a circularinertance tube as follows:

The inertance of a circular tube is 8 where R=radius of the tube, incentimeters, l=efiective length of the tube, that is, length plus endcorrection, incentimeters, and p=density of the medium in the tube, ingrams per cubic centimeter.

Having thus described his invention, what applicant claims is:

1. A transducer comprising a case, a motor cavity within said case, adiaphragm forming one Wall of said motor cavity, a motor positioned inthe motor cavity, drive means connected from said motor to saiddiaphragm, and an inertance tube carried by said diaphragm and havingone end opening into that side of the diaphragm adjacent the motorcavity and the other end opening on the other side of the diaphragm.

2. A transducer comprising a case, a diaphragm mounted in the case andforming a closed motor cavity and a shallow front cavity, a smallopening through the Wall of the case into the front cavity, a motormounted in the closed motor cavity, drive means connected from saidmotor to said diaphragm, and an inertance tube carried by the diaphragmand having one end opening into that side of the diaphragm adjacent themotor cavity and the other end opening on the other side of thediaphragm.

3. An electro-acoustic transducer comprising a case, a diaphragm in saidcase separating a closed cavity from a semi-closed cavity, a motor insaid closed cavity, a drive means connected from said motor to saiddiaphragm, said diaphragm and drive means constituting a movableassembly, and an inertance tube carried by the movable assembly with oneend opening on one side of the diaphragm and the other end opening ontheother side.

4. An electro-acoustic transducer comprising a case, a diaphragm in saidcase separating a closed cavit from a semi-closed cavity, a motor insaid closed cavity, a vibratable armature in said motor, a drive meansconnecting a vibratable portion of the armature to the diaphragm, saiddiaphragm, drive means and armature con stituting a movable assembly,and an inertance tube carried by the movable assembly with one endopening on one side of the diaphragm and the other end opening on theother side.

5. An electro-acoustic transducer comprising a receptacle, a diaphragmmounted in said receptacle to form a sealed cavity therein, a motor anddrive means connecting the motor to the diaphragm mounted in saidcavity, and an inertance tube carried by the diaphragm and lying in aplane substantially parallel to that of the diaphragm, one end of thetube opening on one side of the diaphragm and the other end opening onthe other side.

6. An electro-acoustic transducer comprising an open sided case, adiaphragm disposed across the open side so as to form a fluid tight backcavity, a motor having vibratable armature mounted in the back cavity,and a tube having one end connected to the armature and the other end tothe diaphragm, the armature end of the tube opening into the back cavityand the other end of the tube opening into the outer side of thediaphragm.

7. The transducer of claim 6 in which the case is a closed case so thatthe diaphragm forms both a back cavity and a front cavity and in whichthere is a small opening through the wall of the case into the frontcavity.

8. The transducer of claim 7 in which the opening from the tube into thefront cavity is spaced substantially from the opening through the caseinto the front cavity.

9. The electro-acoustic transducer of claim 8 in which the diaphragm hasa peripheral bend extending continuously around the vibratable centerand close to the periphery of the diaphragm itself, and in which theopening through the diaphragm into the front cavity is outside thisbend.

10. An electro-acoustic transducer comprising a receptacle, a diaphragmmounted in said receptacle to form a closed cavity therein, a motor insaid cavity, and a drive pin connecting the motor to the diaphragm, saiddiaphragm comprising a sheet of vibratory material having a vibratorycenter, an inertance tube carried by said diaphragm concentrically withthe axis through said center and normal to the surface of the diaphragmand extending into the motor cavity, the end of the tube adjacent to thediaphragm opening solely onto the far side of the diaphragm, and theother end of the tube into the motor cavity.

11. An electro-acoustic transducer comprising a case consisting of abottom and side wall, a bulkhead disposed over the open side, anelectro-acoustic motor including a vibratable armature mounted on theinner side of the bulkhead, a reverse peripheral flange on the bulkheadextending outwardly in a direction parallel to the side wall so as toprovide a shallow outwardly directed recess, a diaphragm sealed to theedges of said flange and having a vibratable center, there being anopening through the bulkhead in normal alignment with the vibratablecenter of the diaphragm, a drive rod having one end fastened to thevibratable center of the diaphragm and the other end to a vibratableportion of the armature, a fiat lid seatable over the edge of thebulkhead to provide a shallow front cavity, a small opening through thelid adjacent the diaphragm, and an inertance tube carried by thediaphragm and having the greater portion of its length substantiallyparallel to the plane of the diaphragm with one end opening into thefront cavity and the other end opening into the back cavity.

12. An electro-acoustic transducer comprising a receptacle, a diaphragmmounted in said receptacle to form a closed cavity therein, a motor insaid cavity, and a drive pin connecting the motor to the diaphragm, saiddiaphragm comprising a sheet of vibratory material having a vibratorycenter, a plurality of open-ended, inertance tubes mounted through thesheet at an angle thereto and so spaced around the vibratory center asto maintain dynamic balance of the diaphragm.

13. An electro-acoustic transducer comprising a receptacle to form aclosed cavity therein, a motor in said cavity, and a drive pinconnecting the motor to the diaphragm, said diaphragm comprising a sheetof vibratory material, and an inertance tube having the greater portionof its length lying substantially parallel to the plane of said sheetand carried thereby, one end of the tube opening to one side of thesheet and the other end of the tube opening to the other side of thesheet.

14. An electro-acoustic transducer comprising a receptacle, a diaphragmmounted in said receptacle to form a closed cavity therein, a motor insaid cavity, and a drive pin connecting the motor to the diaphragm, saiddiaphragm comprising two sheets of vibratory material in face-to-facerelationship, a groove on the inner face of one sheet forming anelongated inertance tube therein, there being an opening into one end ofsaid tube through one sheet and into the other end of the tube throughthe other sheet, said groove being in dynamic balance with the vibratorycenter of the diaphragm.

15. An electro-acoustic transducer comprising a receptacle, a diaphragmmounted in said receptacle, to form a closed cavity therein, a motor insaid cavity, and a drive pin connecting the motor to the diaphragm, saiddiaphragm comprising two sheets of vibratory material in face-to-facerelationship, a pair of grooves on the inner surface of one sheet, eachforming an elongated, inertance tube, said grooves being in dynamicbalance with respect 10 to each other and the center of the diaphragm,there being an opening into one end of each groove to one side of thediaphragm and an opening into the other end of each groove to the otherside of the diaphragm.

16. An electro-acoustic transducer comprising a recep tacle, a diaphragmmounted in said receptacle to form a closed cavity therein, a motor insaid cavity, and a drive pin connecting the motor to the diaphragm, saiddia' phragm comprising a sheet of vibratory material having a vibratorycenter, a second sheet of heavier material of like but smallerconfiguration in face-to-face relationship with the first sheet, agroove in the heavier sheet forming an inertance tube between itself andthe lighter sheet, the configuration of the groove being such that thewalls are in dynamic balance with respect to themselves and thevibratory center of the diaphragm, one end of the tube be ing openthrough the heavy sheet and the other end being open through the lightsheet.

17. The electro-acoustic transducer of claim 15 wherein the grooves arearcuate around the center of the diaphragm.

18. The electro-acoustic transducer of claim 15 wherein the grooves arespirals around the center commencing at two outer points apart andterminating at two points closer to the center, the corresponding endsof each of the grooves being open through one sheet of material and theother ends being open through the other sheet of material.

19. An electro-acoustic transducer comprising a receptacle, a diaphragmmounted in said receptacle to form a closed cavity therein, a motor insaid cavity, and a drive means connecting the motor to the diaphragm,said diaphragm comprising a sheet of vibratory material in faceto-facerelationship with a second sheet, an inertance tube formed on the insideface of one sheet, an opening into said tube through the sheet whereinit is formed, and an opening into said tube through the other sheetwhereby the tube opens onto opposite sides of the diaphragm.

20. An electro-acoustic transducer comprising a case, a front cavity anda back cavity therein, a motor in the back cavity, a diaphragmpositioned between the two cavities, and a drive pin connecting themotor to the diaphragm, said diaphragm comprising a sheet of vibratorymaterial having a vibratory center, a plurality of inertance tubes thegreater portion of their length lying substantially parallel to theplane or said sheet and mounted thereon, said tubes extending radiallyfrom the center in dynamic balance with each other and the center, anend of each tube opening to one side of the diaphragm and the other endof each tube opening to the other side of the diaphragm.

21. The electro-acoustic transducer of claim 20 wherein the ends of thetubes nearest the vibratory center of the diaphragm all open on one sideof the diaphragm and the other ends all open on the other side of thediaphragm.

References Cited in the file of this patent UNITED STATES PATENTS1,366,607 Steinberger Ian. 25, 1921 1,847,702 Thuras Mar. 1, 19321,869,178 Thuras July 26, 1932 2,400,281 Anderson May 16, 1946 2,503,857Warnke Apr. 11, 1950 2,789,161 Brennan Apr. 16, 1957

